1. Field of the Invention
The present invention relates to magnetic tunnel junctions and a magnetic random access memory (MRAM) having the magnetic tunnel junctions as memory cells.
2. Background Art
The MRAM is a nonvolatile memory promising as a potential universal memory in terms of high integration, high-speed operation and so forth. As shown in FIG. 1, an MRAM memory cell 100 is configured to have a magnetic tunnel junction 101, such as GMR (Giant magnetoresistance) elements and TMR (Tunnel magnetoresistance) elements, and a selecting transistor 102, which are electrically connected to each other in series. The selecting transistor 102 has a source electrode, a drain electrode and a gate electrode, which are electrically connected to a source line 103, to a bit line 104 via the magnetic tunnel junction 101, and to a word line 105, respectively. The magnetic tunnel junction 101 has a three-layer configuration as a basic configuration, in which a nonmagnetic layer 108 is interposed between two ferromagnetic layers, a first ferromagnetic layer 106 and a second ferromagnetic layer 107. In an example shown in a drawing, the first ferromagnetic layer 106 is a reference layer whose magnetization direction is invariable, whereas the second ferromagnetic layer 107 is a recording layer whose magnetizing direction is variable. The magnetic tunnel junction 101 has low resistance when the magnetization directions of the reference layer and the recording layer are parallel (P state) to each other and has high resistance when they are anti-parallel (AP state) to each other. As shown in Appl. Phys. Lett., 93, 082508 (2008), a resistance variation rate exceeds 600% at room temperature in the case of TMR elements having the nonmagnetic layer 108 made of MgO. Such a large resistance variation rate is known to be seen in the case where coherent tunnel transport via a Δ1 band is used, the tunnel transport being implemented when a ferromagnetic material, including at least one of 3d transition metal elements such as Co and Fe, is applied to the first ferromagnetic layer 106 and to the second ferromagnetic layer 107 and MgO is applied to the nonmagnetic layer 108.
Magnetic tunnel junctions, such as TMR elements, are nonvolatile since information is stored by using the magnetic configuration. The magnetic tunnel junctions are expected not only to be used for the MRAM but also to be applied as memory elements distributed to logic circuits. When the magnetic tunnel junctions are used as memory elements of the MRAM and the like, the resistance variation of the magnetic tunnel junction is made to correspond to “0” and “1” bit information. As a method for writing bit information, a magnetization reversal method by spin-transfer torque has been proposed as shown in J. Magn. Mater., 159, L1-L7 (1996). This method uses a phenomenon of magnetization direction switching induced by spin-transfer torque which is generated by passing current through the magnetic tunnel junction. When current is passed from a reference layer to a recording layer, the magnetization directions of the reference layer and the recording layer become anti-parallel to each other, and thereby bit information is set to 1. When current is passed from the recording layer to the reference layer, the magnetization directions of the reference layer and the recording layer become parallel, and thereby bit information is set to “0.”
In order to implement an MRAM, there are some requirements which should be satisfied simultaneously by the magnetic tunnel junction 101 that is a recording element. Main requirements include (1) high magnetoresistance variation rate (MR ratio), (2) low switching current, and (3) high thermal stability factor. Specific performance requirements which should be satisfied vary depending on application parameters such as an integration density, a minimum, processing size, and working speed. For example, as for the performance requirement (1), the MR ratio needs to be higher as readout speed is increased. This performance requirement also varies depending on the application of the TMR element, i.e., depending on whether the TMR element is used as a memory element compositely mounted with logic circuits or the TMR element is used as a memory element of a single memory device. Generally, the MR ratio is as high as 50% to 100% or more. As for the performance requirement (2), the switching current needs to be lower than the current that can be supplied by the selecting transistor. The smaller selecting transistor 102 provides lower drive current. Therefore, when the selecting transistor 102 becomes smaller, the required switching current needs to be equal to or lower than the drive current on a constant basis. Further, the condition (3) relates to record retention time of the magnetic tunnel junction 101 and to write disturb therein. For ensuring ten years or more record retention time and preventing write disturb, a 1-bit TMR element needs a thermal stability factor of 40 or more. As the MRAM has a larger capacity, a required thermal stability factor is increased. In order to implement a Gbit-class MRAM, the thermal stability factor needs to be 70 to 80 or more.
In order to meet these requirements, inventors of the present invention prepared a component for use at least in either one of the first ferromagnetic layer 106 and the second ferromagnetic layer 107, which constitute the magnetic tunnel junction 101 of FIG. 1, from a material including at least one kind of 3d transition metals, such as Co and Fe. When magnetic tunnel junctions are manufactured with a material which includes at least one kind of 3d transition metals such as Co and Fe and which is crystallized into a bee structure by thermal treatment, the magnetization direction of the ferromagnetic layers is generally parallel to a film plane. However, as shown in Nature Mater., 9, 721 (2010), the inventors of the present invention have developed a technology to control the film thickness of the ferromagnetic layers within 3 nm or less and to orient the magnetization direction perpendicular to the film plane. The magnetization direction perpendicular to the film plane is presumed to be provided by interface magnetic anisotropy which is induced at the interface between MgO used as a material of the nomnagnetic layer 108 and a material of the ferromagnetic layer including at least one kind of 3d transition metals such as Co and Fe. By using the interface magnetic anisotropy effectively, the perpendicular magnetic anisotropy is induced. With use of this technology, the aforementioned MR ratio of 100% or higher, the switching current lower than the drive current of the selecting transistor 102, and the thermal stability factor of 40 or more are implemented.